An equalizer is an invaluable tool in the process of music production. It can be used to remove or boost certain frequencies in the mix to improve the sonic quality of the track. It can also be used creatively to introduce new sounds that provide a desired character to the track. 

Like all tools available during music production, an equalizer needs to be used only where needed.

The following article provides you with a very informative list of frequency ranges that can be referred to during the process of mixing in order to enhance your track.

The frequencies mentioned only provide you with a direction in which you need to look. Use your ears and tweak as needed to get the desired results . Good luck and happy mixing!

http://www.songstuff.com/recording/article/eq_frequencies/

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